<html window-frame="solid-with-shadow">
  <head>
    <title>?</title>
    <style>

body { margin:0; }

html > button#close {
  appearance:none;
  display:block;
  position:absolute; right:4px; top:4px; 
  behavior:clickable;
  size:16px;
  padding:0;
  border-radius:8px;
  background: transparent no-repeat 50% 50%;
  background-image: url(stock:cross-x);
  foreground-size: 8px;
  fill:#fff;
  visibility:hidden;
  transition: visibility linear 200ms, background-color linear 200ms;
}

html:hover > button#close {
  visibility:visible; 
  background-color: #c00;
}

    </style>
    <script type="module">
document.body.content(Window.this.parameters);
    </script>
  </head>
  <body></body>
  <button #close role="window-close" />
</html>